Cathode



July 16, 1940.

F. W. LYLE CATHODE Filed March 19, 1958 INVENTOR flor/CK /1/. Z y/e.

WITNESSES:

ATTORN y Patented July 16, 1940 v UNITED-STATES PATENT OFFICE CATHODE Frederick W. Lyle, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania ApplicationMarch 19, 1938, Serial No. 196,955 4 Claims. (Cl. 250-275) My invention relates to incandescent helically- It has been customary in the prior art to employ wound metallic ribbons and, in particular, to such a ribbon having the same thickness throughout ribbons used as electron-emissive cathodes in its width, but it can be shown that helices made electrical discharge devices. up from such ribbon tend to operate with a higher "i One object of my invention is to insure that a temperature at their radially inward edge than 5 helical edgewound metallic ribbon, which is at their radially outward edge. Since electron heated by electric current flowing therethrough, emissivity in general varies with a rather high shall operate as nearly as possible without variapower of the temperature of the emitting surtion in temperature radially from the inner to the face, it is desirable that all portions of a cathode o outer edge of the ribbon. surface shall operate at as high a temperature as Another object of my invention is to insure that the material is able to withstand without being helical edgewound metallic ribbons employed as progressively destroyed by high temperature. It directly-heated electron-emissive cathodes for follows accordingly that the above-mentioned electrical discharge devices shall vary as little as tendency of edgewound ribbon cathodes to operpossible in electron emissivity between radially ate with ahigher temperature at their inner edges inward and radially outward sections of the than at their outer edges is a defect which results cathode surface. in an inefficient use of the radially outer portion Other objects of my invention will become of the electron-emitting surface. apparent from the following specification, taken In accordance with my invention, I so modify :0 in connection with the drawing in which: the form of the ribbon that its radially inner edge ,Figure 1 shows, in side elevation, a typical helioperates at approximately the same temperature cal edgewound ribbon cathode for an electrical as its outer edge. One expedient which I employ discharge device; s s for doing this is to roll the ribbon with a cross- Fig.2 shows an end elevation of the same cathsection, illustrated in Fig. 3; namely, with a ode; slightly wedge-shaped cross-section, so that the g Fig. 3 shows a cross-section of the cathode inner edge of the helix is thinner than the outer ribbon along line III1II of Fig. 2. edge. A ribbon of this cross-sectional form is 4 Shows an end w of or Similarathode adapted tobe welded to terminal-studs, the ends embodying a modified form of terminal connecof which are perpendicular to the axis of the tion; and I r ribbon, as illustrated in Fig. 2. Referring to the 30 Fig. 5 shows, in side elevation, at short section of latter figure, distances from the axis of the helix one end of t e cathod illustratedin may be denoted by the letter 1", and it will be One form of i cande t o Y-emisnoted that, in the case of an uncrimped ribbon, if sive cathode which has been very successfully used a current path of infinitesimal width dr at a for e ct c discharge tubes of comparatively distance r from the axis be considered, the length 35 l r curr n rating mpr s a i bon f som of this current path from one terminal to the suitable metal I, such as nickel, or a nickel-cobaltth r 111 be 2,,N1'+2L, where N is the number of titanium al y n w to the trade as Konal, turns of the helix and L is the distance from the which is wound into a helix with the broad dimennd of th t r in lt d to th plane of t 1 sion of the bb approximately perpendicular to cal axis. In the usual filament, N will be so large 40 the axis of the helix. The ends of the ribbon I compared with L that the ter 21, may b are usua y w d d t vy m all c l ads 2 neglected; In other words, current passing from which are Sealed t o the glass Wall of on one terminal-stud to the other through the curevacuated Cont and which pp electric rent path of width dr at a distance r from the axis current to heat the helix to an incandescent temtra rses a. path of approximate length 2,,Nr. It 45 p atu Before installation in t tube, the is obvious that the length of this path is greater helix is sua y Coated With a mixture of barium for those portions of the ribbon where r is large and strontium oxides or other materials which than f r those portions where T is smalL are well known in the art to freely emit electrons In typical filament, of t prior t in which when raised 130 an elevated temperature. It has the thickness of the ribbon was the same for 3,11 50 been found that both t W d of t e helix values of r, it is clear that currents traversing and its subsequent operation are improved if, paths of Width dr which were near the inner edge before winding, the ribbon is slightly crimped, of the ribbon, and for which-r was consequently which may be done by passing it between a pair small, traversed paths of smaller electrical resist- 56 of toothed rollers. ance than currents traversing paths of similar width 011' near the outer edge of the ribbon. In

spaced, it may be that the tendency of adjacent other words, current density was greater in the radially inward portions of the ribbon than in the radially outward portions of the ribbon, and in consequence of this greater density of heating current, the radially inward portions of the ribbon tended to run at a higher temperature than the radially outward portions of the ribbon.

Where the axial distance between turns of the ribbon is comparatively large, it is apparent that the same temperature would be obtained forall.

ribbon at radial distance r, and N is the number of turns, it is obvious that in order to make A constant for all values of r, t, the thickness of I the ribbon, must be proportional to r. In other words, the cross-section of the ribbon should be i a truncated triangle having a thickness at its inner edge in the same ratio to its thickness at the outer edge as the inner radius of the helix is to the outer radius of the helix.

Where the turns of the helix are more closely turns to heat one another will cause a higher temperature at the inner edge of the helix than at the outer edge, with the result that the inner edge must be made somewhat thinner than the proportion outlined in the foregoing paragraph would indicate. Since calculation of such a problem in heat flow is quite complicated, it is probably easier as a practical matter to determine the proper relative thicknesses of the inner and outer edges of the helix in the case of closely wound filaments by trial and experiment. The general principle may, however, be followed that, where a given cross-section is found to produce too high atemperature in the radially inward portions of the ribbon, the inner edge of the ribbon should be thinned down; and vice versa.

Where the distance L in Fig. 2 is notnegligible compared with 211'N1, the thickness of the ribbon should be proportional to 21rNT-l-2L.

tain cases where it is desirable, and this is illustrated in Figs. 4 and 5. Instead of making the ends of the terminal-studs to which the ribbon is welded perpendicular to the axis of the ribbon, they may be slanted in the manner shown in Fig. 4 to such a degree that the length of the path of widthdr at a distance r from the axis of the helix is the same for all values of r, in which purpose aswell as separately.

While I have described a particular embodiment of my invention, it is obvious that the principles thereof are of broader application in ways which will be obvious to those skilled in the art. I accordingly intend that the following claims shall be only limited insofar as is made neces sary by their express terms taken in conjunction with the prior art.

I claim as my invention:

.1. An incandescent cathode comprising a helical edgewound metallic ribbon, the thickness of the ribbon at any point being proportional to the radial distance of said point from the axis of the. helix.

2. An incandescent cathode comprising a helical edgewound metallic ribbon, the thickness of the ribbon at any point being proportional to the radial distance of said point from the axis of the helix aiiixed to terminal leads at junctions which are approximately perpendicular to the axis of the ribbon.

3. An incandescent directly-heated cathode comprising a helical edgewound ribbon having straight portions at the ends of the ribbon aflixed to terminal-studs at junctions substantially perpendicular to the axis of the ribbon, the length of each said straight portion being L, the helix having N turns, and the thickness of theribbon at distance r from the axis of the helix being proportional to. 27rNT+2L.

4. An incandescent directly-heated cathode comprising a helical edgewound ribbon affixed to terminal posts at junctions forming an angle with the axis of the ribbon, the length of any current path from terminal post to terminal post through the ribbon at a distance r from the axis the ribbon at the distance 1'.

FREDERICK W. LYLE. 

